The invention relates to a photometric apparatus for a camera, and more particularly, to a photometric apparatus for a camera which develops a logarithmic compression signal for an exposure display and an integration signal for an exposure control.
There has been proposed such a photometric apparatus of the type that is constructed as shown in FIG. 1 (Japanese Laid Open Patent Publication No. 54-48235). The apparatus shown in FIG. 1 includes an operational amplifier 1 which has an inverting input terminal .crclbar. and a non-inverting input terminal .sym. to which a reference voltage V.sub.REF is applied. A photoelectric transducer element 2 such as a photodiode is connected betwen the input terminals .crclbar. and .sym.. An output terminal of the amplifier 1 is connected to a contact terminal c of a timing switch 3 whose connection is switched from a contact terminal a to a contact terminal b in synchronism with the running of a first shutter blind. A diode 4 for logarithmic compression is connected between the inverting terminal .crclbar. of the amplifier 1 and the contact terminal a. An integration capacitor 5 is connected between the inverting terminal .crclbar. of the amplifier 1 and the contact terminal b. Resistors 6 and 7 are connected between the non-inverting terminal .sym. of the amplifier 1 and contact terminals a and b, respectively.
With the construction of the photometric apparatus described above, when a photocurrent flows through the transducer element 2, a logarithmic compression signal for an exposure display to be fed to a view finder display system is derived from an output terminal 8 connected to the contact terminal a. When the contact terminal c is connected to the contact terminal b in synchronism with running of the first shutter blind upon shutter releasing, an integration signal for an exposure control is derived from an output terminal 9 connected to the contact terminal b.
In the photometric apparatus shown above, however, the mechanical type timing switch 3 and the externally disposed integration capacitor 5 are used and therefore there are difficulties in making it into an integrated circuit (IC) and a limitation upon its miniaturization. In addition, when the mechanical type timing switch 3, which is a transfer switch, is employed, there is an open connection interval in which the contact terminal c is connected to none of the other contact terminals as the contact terminal c changes connections from the contact terminal a to the contact terminal b and thereby during which a feedback loop of the amplifier 1 is disconnected, resulting in no feedback. At this moment, since an output of the amplifier 1 becomes unstable and both terminal voltages across the integration capacitor 5 are not maintained at zero volts, a malfunction may be caused by unstable circuit behavior such as a time lag of an integration initiation. Therefore, a particular means is generally required to compensate for the above noted problem. In addition, since the diode 4 for logarithmic compression is disposed in the feedback loop between the output terminal and the inverting input terminal .crclbar. of the amplifier 1, it needs a time duration to charge the diode 4 with its junction capacitance, especially a few seconds from turning a power supply on to obtaining a proper logarithmic compression output when light from an object being photographed is poor. Accordingly, it is to be noted that in such a case also a particular measure is practically required to reduce the time duration in prior art photometric apparatus.